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Chapter 17: Aldehydes
and Ketones. Nucleophilic Addition to C=O |
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Spectroscopic Analysis
of Aldehydes
Absorbance (cm-1)
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Interpretation
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1710 -1750
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C=O stretch
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~2720
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C-H stretch of aldehydes
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~2820
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C-H stretch of aldehydes
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- 1H
NMR - the CH=O unit of aldehydes is the most easily recognised.
Resonance (ppm)
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Interpretation
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9.0 -10.0
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CH=O (deshielding
due to O and C=C)
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~2.4
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CH2C=O
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~2.0
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CH3C=O
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-coupling (J, Hz) between protons
in HC-CH=O is much smaller than HC-CH=C.
- 13C
NMR
C=O typically 190-220 ppm (deshielding due to O)
- minimal intensity, characteristic
of C's with no attached H's
- UV-VIS
two absorption maxima p to p*
(< 200 nm) n to p* (> 200 nm)
- p
electron from p
of C=O
- n electron from O lone
pair
- p*
antibonding C=O
- Mass Spectrometry
Molecular ion, M+, is prominent and typically an M-1 peak.
Loss of alkyl radical leads to the formation of acyl cations (acylium ions).